Liquid-liquid extraction is used generally in metal separation processes, allowing metals to be extracted from aqueous solution using organic extraction solutions. An extraction solution consists of an extraction reagent and a hydrocarbon solvent. The extraction reagent is generally diluted in the kind of hydrocarbon solvent that dissolves into an aqueous solution or evaporates into air as little as possible in process conditions.
The composition of both the active extraction reagent and its hydrocarbon solvent in the extraction solution has been found to change during long-term industrial use. As a result, the metal-binding power of some extraction reagents may have worsened. In particular this has been observed in copper extraction processes and nickel extraction processes using various reagents based on hydroxyoxime derivatives. The reagents mentioned are also used for the extraction of certain other metals and metalloids (e.g. palladium and germanium) as well as in some synergistic extraction reagent mixtures to modify the selectivity for different metals. Changes have also been found to occur in the composition of the hydrocarbon solvent. It is known that hydrocarbon solvents oxidise slowly and generate among other things surface-active long-chain carboxylic acids.
It has been known previously that a hydroxyoxime reagent used in extraction, which has degraded in the hydrolysis reaction into aldehyde or ketone, may be reoximated using hydroxylamine (NH2OH) or a salt thereof. The reoximation reaction takes place as follows:R1R2C═O(ketone)+NH2OH→R1R2C═N—OH(ketoxime)+H2O
If R2=H in the formula, the source material in question is some aldehyde and the product the corresponding aldoxime. If R2 is for example an alkyl or aryl group, it concerns a ketone and ketoxime. This same equilibrium reaction from right to left, in other words acid-catalysed hydrolysis, is one of the decomposition reactions that occurs when hydroxyoxime is used as the extraction reagent in the extraction process. However, it is known that hydroxyoximes also decompose e.g. in oxidation reactions.
It is disclosed in U.S. Pat. No. 4,104,359 that organic sulphonic acid causes the degradation of an α-hydroxyoxime reagent in the organic phase. According to the patent, α-hydroxyoxime can be reoximated in the above-mentioned mixture directly using a solid hydroxylamine salt. According to the patent, the method can also be used for β-hydroxyoximes, which are ketoximes (current copper extraction reagents often belong to the aldoximes). The patent also mentions that alternatively a saturated aqueous solution of hydroxylamine may be used for the purpose and the process may be performed for instance in a mixer-settler-type of extraction cell. However, there are no practical examples in the patent of the implementation in which a saturated aqueous solution is used directly. As shown later in the examples of this patent application, the method accordant with the U.S. patent would be very slow and the conversion in it would remain low. In addition, undesirable reactions also take place in reoximation and consequently, the settling of the phases for instance is very slow. The implementation method accordant with this reference publication without a purification step is therefore not satisfactory technically.
A method is disclosed in U.S. pat. No. 5,993,757 that also covers β-hydroxyoximes, which may be either ketoximes or aldoximes. According to the method, the hydroxyoxime extractant that has decomposed into ketone or aldehyde is reoximated using an aqueous solution of a hydroxylamine salt. The patent differs from the earlier patent so that one of the patent claims is for a distillation purification stage before reoximation and the use of a phase transfer catalyst in the reoximation reaction. The reaction time is 5 - 36 hours. As stated later in the examples of this patent application, both distillation and the use of a phase transfer catalyst can be avoided. Likewise the reaction can be implemented considerably faster and operated in a continuous flow mode.